The present invention relates generally to radiographic imaging and, more particularly, to an x-ray filter having dynamically displaceable x-ray attenuating fluid.
Typically, in radiographic systems, an x-ray source emits x-rays toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” may be interchangeably used to describe anything capable of being imaged. The x-ray beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the radiation beam received at the detector array is typically dependent upon the attenuation of the x-rays through the scanned object. Each detector element of the detector array produces a separate signal indicative of the attenuated beam received by each detector element. The signals are transmitted to a data processing system for analysis and further processing which ultimately produces an image. Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
In a similar fashion, radiation detectors are employed in emission imaging systems such as used in nuclear medicine (NM) gamma cameras and Positron Emission Tomography (PET) systems. In these systems, the source of radiation is no longer an x-ray source, rather it is a radiopharmaceutical introduced into the body being examined. In these systems each detector of the array produces a signal in relation to the localized intensity of the radiopharmaceutical concentration in the object. Similar to conventional x-ray imaging, the strength of the emission signal is also attenuated by the inter-lying body parts. Each detector element of the detector array produces a separate signal indicative of the emitted beam received by each detector element. The signals are transmitted to a data processing system for analysis and further processing which ultimately produces an image.
In most computed tomography (CT) imaging systems, the x-ray source and the detector array are rotated about a gantry encompassing an imaging volume around the subject. X-ray sources typically include x-ray tubes, which emit the x-rays as a fan or cone beam from the anode focal point. X-ray detector assemblies typically include a collimator for reducing scattered x-ray photons from reaching the detector, a scintillator adjacent to the collimator for converting x-rays to light energy, and a photodiode adjacent to the scintillator for receiving the light energy and producing electrical signals therefrom. Typically, each scintillator of a scintillator array converts x-rays to light energy. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data acquisition system and then to the processing system for image reconstruction.
For radiographic imaging, such as x-ray imaging and computed tomography, x-ray exposure of a subject is always a concern. The amount of irradiation seen by a scan subject is generally referenced as “x-ray dose” and is factor that is paramount in prescribing a scan. That is, image quality is greatly influenced by the x-ray dose during data acquisition. In this regard, at higher dose levels, SNR is greater, which leads to better image quality. At higher dosage levels, however, the subject is exposed to greater amounts of irradiation. As there are strict guidelines to the amount of irradiation that a subject can experience, practitioners must limit x-ray dose and, as a result, sacrifice image quality. So, when prescribing a scan, practitioners choose a dosage level that will provide the best image quality without exceeding mandated irradiation levels.
Adding to the difficulty in setting dosage levels is that subjects, such as medical patients, lack a uniform thickness. This is particularly problematic for CT systems where each view in a complete scan rotation presents a different angle of x-ray illumination to the subject. As such, it is difficult to optimize x-ray dose on a view-by-view basis. Moreover, subjects generally have variable attenuation characteristics across a given field-of-view. More specifically and in the context of medical patients, outside the skin line, there is no attenuation and the full flux of the x-ray beam is incident on the x-ray detector. Just within the skin line, the attenuation is much larger relative to outside the skin line and, as a result, fewer x-rays reach the detector. Since the x-ray source during a scan is operated so that the number of x-ray photons within the center of the field-of-view is sufficient to create an image, e.g., low noise, the excessive photons at the skin line interface are unnecessary for image quality. Typically, without a proper x-ray management device, this increased photon number at the skin interface imports additional dose to the subject and results in x-ray scatter into the imaged region. Therefore, it has been desirable to reduce the x-ray flux outside the imaging volume and to do so with each view. This is increasingly desirable for detectors that saturate at low x-ray flux levels.
That is, energy discriminatory and photon counting detectors have a much lower saturation limit than conventional, energy integrating detectors. Despite the drawbacks associated with low flux saturation, photon counting detectors are desirable in order to ascertain energy information of the x-rays detected by the detectors which can be used for material discrimination. However, because direct conversion, photon counting detectors typically having a low flux rate limit, such as about 1 million cnts/sec/mm2), their use has been significantly limited.
Additionally, the concerns of increased x-ray photons are not isolated to the skin line interface. If the field-of-view is relatively uniform for all views, then abrupt x-ray flux changes will only be experienced at the edge of the field-of-view. However, for medical imaging, such a case is rare. Typically, the asymmetry of the object being imaged results in very different flux profiles for different views. The impact of this variance can be mitigated if the center of the field-of-view has the thickest cross-section and the field-of-view boundary is marked by relatively high flux transition. However, if the object to be imaged is off-centered, then one side of the detector will see much higher flux rates than other side of the detector and this will change on a per view basis. It is also possible to have a great degree of variability within the field-of-view if the internal composition of the object causes large variations in signal level across the field-of-view. One skilled in the art will appreciate that numerous flux conditions other than those presented above can be encountered and lead to detector saturation and/or areas of unneeded x-ray flux.
Heretofore, fixed shaped filters have been used to selectively attenuate x-rays at the edges of the field-of-view more than at the center of the field-of-view. A common fixed shape filter is generally referenced as a “bowtie” filter and is commonly used in CT systems to compensate for the thickest-in-the-middle characteristic of most medical patients. Known bowtie filters, however, are fixed in their shape and, thus, to accommodate the variations that could be encountered in a scan subject population, CT systems are generally equipped with several discrete bowtie filters. Not only does this lead to increased costs, but because the filters are static, the filters cannot be optimized for dynamic changes that occur as the source-detector rotates during a CT scan. In this regard, known bowties are ineffective in preventing detector saturation across the several views of a CT scan and can result in increased dosage levels to maintain image quality.
It would therefore be desirable to design an x-ray flux management device that is effective in reducing detector saturation under high x-ray flux conditions while not compromising data acquisition under low x-ray flux conditions. It would also be desirable to have such an x-ray flux management device that provides further optimization of radiation dose during a scan.